This invention relates to hydraulic systems wherein hydraulic work performing means are subject to varying load requirements. More particularly, the invention relates to energy conservation in such systems. The invention is particularly well suited for, but not limited to, use in electric lift trucks.
Prior art of possible relevance includes U.S. Letters Pat. No. 3,864,911 issued Feb. 11, 1975 to Gellatly et al.
Air pollution problems have resulted in a reawakening of interest in electrically driven vehicles by reason of their avoidance of emission problems. Successful operation of such vehicles, however, has been hindered to a great degree by energy considerations. As is well known, such vehicles operate off of direct current supplied by batteries carried with the vehicle and their range of use is dependent upon the energy storage capacity of the batteries which, in turn, is dependent upon the number and type of batteries which the vehicle may carry.
In certain applications for electric vehicles, duties other than propulsion are required of the electrical system. For example, in electric lift trucks, ultimately the energy provided by the batteries is employed to raise loads carried by a fork or the like on the vehicle mast as well as to tilt the mast at various times during operation. In general, such lift trucks employ hydraulic cylinders for effecting the lift and tilt operations and the cylinders are provided with hydraulic fluid under pressure from a pump driven by an electric motor.
In most operations of lift trucks, considerably greater energy is expended during a lifting operation than is expended when the mast is merely being tilted. Thus, when such a lift truck is provided with a pump having a sufficient capacity to properly operate the lift cylinder, the use of the output of such a pump for tilting operations represents a waste of the capacity thereof and a concomitant waste of the battery energy.